Process for starting and performing technical processes using electrical glow discharges



Oct. 10, 1961 B. BERGHAUS ET AL 3,004,133v

PROCESS F OR STARTING AND PERFORMING TECHNICAL PROCESSES USING ELECTRICAL GLOW DISCHARGES Original Filed Dec. 8, 1954 2 Sheets-Sheet 1 C ur-ren-f Fig.2

Oct. 10, 1961 B. BERGHAUS ET AL 3,004,133

PROCESS FOR STARTING AND PERFORMING TECHNICAL PROCESSES USING ELECTRICAL GLOW DISCHARGES Original Filed Dec. 8, 1954 2 Sheets-Sheet} United States Patent 3,004,133 PROCESS FOR STARTING AND PERFORMING TECHNICAL PROCESSES USING ELECTRICAL GLOW DISCHARGES Bernhard Berghaus and Hans Bucek, Zurich, Switzerland, assignors to Elektrophysikalische Anstalt Bernhard Berghaus, Vaduz, Liechtenstein Original application Dec. 8, 1954, Ser. No. 473,895. Divided and this application Oct. 8, 1958, Ser. No.

8 Claims. (Cl. 21950) The present invention is a division of copending application Serial No. 473,895, filed December 8, 1954, and a continuation-in-part of copending application Serial No. 579,934, filed April 23, 1956, both now abandoned, and relates to a process of performing technical processes using electrical glow discharges.

In the industrial performance of such technical glow discharge processes difficulties have frequently arisen and undesirable instabilities of discharge have been observed.

The present invention has for its object the provision of a uniform technical rule to which all determinant factors for the starting and performance of such technical glow discharge processes are clearly subordinated. The process according to the present invention is characterized by the fact that a gas discharge condition is obtained and maintained at least in the immediate proximity of the surfaces participating in the process and at least after the final discharge stage has been reached, in which condition the thermal electron current emitted by the surfaces, which disturbs the balance of discharge in the cathode fall space, is over-compensated by the ionic current to the said surfaces. A cathode fall is thereby maintained in this space and transition into unstable ranges of the discharge characteristic and concentration of discharge to a focal area avoided. The total discharge potential is larger than double the are voltage of the discharge space.

A number of embodiments of the process according to this invention are shown in the attached drawings in which:

FIG. 1 is a typical current/voltage characteristic of glow discharges compared with a characteristic obtainable by the process according to the invention;

FIG. 2 is a diagrammatic view of thermal space currents I dependent on absolute temperature;

FIG. 3 is a diagrammatic view of the region of the process according to this invention in a special system of co-ordinates.

As is well known, the current/voltage characteristic 65 of, for instance, an electrical gas discharge as hitherto known according to FIGURE 1, operated by a direct current voltage, shows a so-called norma region X and a following abnormal region Y of higher voltages, whereby the abnormal region Y is followed, with a further voltage and current increase, by a falling characteristic leading to the point 67, where the glow discharge passes into an arc discharge.

The current/ voltage diagram shown in FIGURE 1 and its characteristic 65 show the typical course of glow discharges in the case of direct current voltage according to the present state of the art and science (see, for instance, Dosse, Nierdel The Electric Current in High Vacuum and in Gases, Hirzel, 1945, p. 317, and Loeb, Fundamental Process of Electrical Discharges in Gases, published by Wiley, 1947 pp. 606 and 608).

The normal region X of the discharge terminates at that current at which the voltage carrying parts of the electrodes are completely covered by the glow discharge. With further voltage increase, the voltage and the discharge current increase, whereby the increasing voltage which, as is well known, has a substantial effect concentration on the so-called cathode drop, immediately before the negative electrode, so that the positive gas ions impinge upon the electrode surface which increased kinetic energy. In operation with direct current voltage, this effect takes place continuously at the cathode, whilst in operation with alternating current each electrode becomes a cathode during each half period.

In the space of the cathode drop, when the glow discharge is not disturbed, equilibrium takes place between the current of ions to the electrode surface and the electrons which are there released. The increase in energy of the impacting ions which takes place with increase in voltage heats up the corresponding electrode, which leads to a thermal emission of electrons from the metal of the electrode. This thermal emission current of negative electrons, and other but little explained emission processes taking place at the electrode in mutual reaction with the surrounding layer of gas, may lead to a contraction of the discharge to a burning spot and to ignition of an are between the electrode and the adjacent counterelectrode. This transition into the arc discharge corresponds to the point 67 of the characteristic 65, which always lies at that point where the glow discharge cathode drop is caused to disappear to a large extent by the emission of electrons from the metal of the electrode. The total discharge voltage of the are discharge is always less than half the glow discharge voltage in the case of operation in the normal region X of the characteristic. It is to be pointed out that the physical conditions in the case of powerful glow discharges are not yet completely elucidated. For instance, it is possible that another emission takes place before the thermal one, for instance a secondary emission of electrons, a field emission, and so on. Discharges without a well defined burning spot, have also been made known by publications, for which, however, as compared with the glow discharge voltage, much lower working voltages are just characteristic as for the arc discharge contracted in a burning spot. The above explained transition from a glow to an arc discharge represents a possible explanation according to the present state of the art, but, as regards the process according to the invention, which has been developed by experimental investigations, serves only as a working hypothesis.

As regards industrial processes by means of glow discharge, transition into an arc discharge must be avoided in all circumstances, since the same always causes local overheating at individual points of the electrode surfaces and does not allow of any uniform and repeatable process of the known kind to be carried out. The increase in the density of the energy of the glow discharges was hitherto limited by the heating of the electrodes thereby arising and by their thermal emission of electrons, which necessarily led to a transition into an arc discharge with more or less concentrated contraction of the discharge on limited electrode regions, with a simultaneous drop of the discharge voltage to values much below volts. Thus, it was not possible in the past, in the case of an increase in the energy transformation of a glow discharge, to avoid the unstable transition region of the discharge characteristic from the glow to the arc discharge.

The present process thereby enables to obtain any desired increase in the energy transformation of the glow discharge into heat up to any desired temperature of the electrode surfaces participating in the process whilst ensuring of a continuous and continuously rising characteristic, as is indicated for instance by 66 in FIGURE 1, being obtained. This is rendered possible by the fact that the electron emission of all metals and their compounds has for any temperature a definite value which cannot be exceeded. The emission per unit of surface is known exactly for most chemically pure substances in dependence upon the temperature. If in the case of a predetermined desired temperature of the surfaces participating in the process one can produce and maintain in their immediate proximity a state of gas discharge at which the current of positive ions flowing towards the cathode is higher than that required for the balance of the discharge, preferably even a multiple of the electronic current emitted by the corresponding surfaces, then the disturbance of the discharge balance in the cathode drop space by the electron current that is emitted cannot exert a dominating influence, that is to say, tend to effect transition to an arc discharge.

However, one must in this case be sure that the ionic current has the required value at all the points of the surfaces participating in the process, so that also in the case of sudden occurrence of strongly emitting points, of gas outbursts, and local overheating resulting therefrom, up to white heat, it will at these points be higher than the emission current. Only when this is the case, it is impossible for such disturbing places on the electrode surfaces to have a marked influence on the cathode drop space.

The expected thermal electron emission of the surfaces participating in the process has, therefore, to be approximately estimated in advance for the processes that come into question in practice, based on the particular material and the desired temperature. The total current of the gas discharge has then to be adjusted to at least double the value, but preferably a substantially higher value than that, of this estimated emission current, in order to compensate by the ionic current the undesirable influence of the electronic current upon the cathode drop space.

The thermal emission Ie can be calculated for chemically pure metals and metal compounds by means of the so-called Richardson formula, and be entered, for instance of platinum (Pt), tungsten (W), tantalum (Ta), tungsten with thorium (Wo+Th), and barium oxide (BaO), in the diagram shown in FIGURE 2, as a function of the absolute temperature T. As is well known,

in the case of chemically pure metals, an appreciable emission current is obtained only at relatively high temperatures, over about l000 K., whilst, in the case of metal oxides and certain alloys a thermal emission takes place which is greater by one or several orders of values. However, it has to be taken into consideration that in practice the industrial glow discharge processes have to be carried out almost exclusively in connection with alloys or workpieces which are not chemically pure, at least on the surface, and that in the case of reduction and melting processes even metal oxides have to be treated. Therefore, when estimating the maximum pos sible thermal emission current one cannot start with the values applicable to chemically pure metals.

The diagram of FIGURE 2 shows three straight lines 68a, 68b, 680 which are used for estimating the maximum possible emission current Ie for processes according to the present invention. The line 68a represents the minimum current density Ie which is to be entered as a possible emission current also in the case of any low temperature of the surfaces participating in the process, in order to render ineffective any possible local surface defects connected with a strong emission. In the case of processes in which metals and metal alloys are treated at approximately 1500" K., Ie is estimated by the line 68b. If thereby the total current is made equal or greater than this value Ie, a stable glow discharge can be ensured for any desired temperature. If, on the contrary, metal oxides are present in the surfaces participating in the process, it is advisable to provide an emission current Ie corresponding to the course of the straight line 68c. However, it is to be pointed out, that it is only a question of purely empirical calculations for Ie, and that in no circumstances can it be asserted that these Values would justify an interpretation of the actual processes. On the other hand, they represent values as certained by experience in order to keep glow discharges stable in region of characteristic lines and under working conditions in which, hitherto, such discharge phenomena were unknown. The novelty of this discharge region follows also from the assertion that thereby the proportion of atomic gases in the glow discharge zone is larger than was to be expected.

The adjustment of the required total current is effected mainly by an increase in the gas pressure in the discharge vessel, which has always to be so high that the total current is equal to or greater than the estimated value Ie. The required working voltage is thereby always within the limit values indicated in FIGURE 1, viz. Ul=l00 v. and U2=l,800 v., generally even in the range from U3=200 v. to U4=900 v., so that the present discharge technique can be designated as a domain of heavy current and low voltage glow discharges. This region of the glow discharge thus difiers fundamentally from the glow discharge processes hitherto proposed with a few thousand volts, which were always carried out with smaller ion densities.

The fact that the given rules enable glow processes to be carried out under very extreme conditions is illustrated by an arrangement whose behavior was viewed through an observation window in the wall of a metal receptacle during the operation. In this case one electrode consisted of a molybdenum pipe having a diameter of about 8 mms. and a length of 50 mms. viz. a surface of 14 cm. and the other electrode was a metal bolt, about 14 mms. away therefrom. The 50 Hz. alternating current voltage at the electrodes was about 700 volts and the receptacle contained hydrogen, at a pressure of 9 mm. Hg. The molybdenum pipe showed, in the discharge state, a temperature of about 2000 C. also in the case of an energy density corresponding to a radiation of about 50 watt/cm. on the outer surface of 7 cm. thus a total output of about 350 watts. At the mentioned temperature, the thermal emission current for molybdenum was, according to FIGURE 2, about 4 Ina/cm. (2,273 K.), altogether about 56 ma. On the other hand, the total current heating the molybdenum pipe is about 500 ma., viz. about 10 times greater than the emission current.

In this gas discharge condition in the immediate proximity of the surface of the molybdenum pipe, there was an almost undisturbed cathode drop space, and accordingly a stable glow discharge, which resulted in a very high energy density of about 50 watts/cm. In spite of the very strong thermal emission, there was no tendency to pass into an arc discharge. No difliculty was encountered in increasing the energy transformation on the surface of the molybdenum pipe with a further increase in pressure and a smaller voltage increase so that the molybdenum pipe melted (about 2700 C.) without the glow discharge becoming unstable.

Thus, in the present process, the pressure p, and also the voltage U of the discharge gap cannot be chosen arbitrarily, but corresponding values have to be chosen, in order to be able to ensure a stable condition of discharge. This can be seen in the diagram shown in FIG- URE 4, in which (U) is represented as abscisse and (U.p) as an ordanate. These two coordinates first appear to be arbitrary values, but taken together with the known valid laws of similarity concerning gas discharges, they have a physical meaning. The diagram shown in FIGURE 4 serves only to indicate the exact limitation of the admissible range of operation of the process to be carried out according to the present invention. The voltage U is given in volt/cm. actually as a field intensity, but, for the sake of simplicity, the numerical value of the total voltage of the discharge gap is inserted, since in this case only the space in the immediate proximity of the surfaces participating in the process, on which almost the total voltage U is concentrated is essential. i

is the current density amp./cm. and the pressure p is indicated in mm. Hg.

The diagram shown in FIGURE 4 relates to examples of the working characteristics of the processes mentioned in the following table, wherein the first three processes were found to be of very little advantage as regards stability and from an economic point of view.

Gas Pressure, mm. Hg

Process, Voltage U Surface,

Number Value Volt Kind The working range used for the present process is characterized by P equal or smaller than 250 g-p equal to or smaller than 5000 Within this range, indicated in the diagram by the lines 80 and 811, lie all the working characteristics or data of the glow discharge processes according to the above described rules. The range given in the examples 73 to 79 encompasses an output of the discharge of 300 watts to 33,000 watts and a current density i of 0.5 to 120 ma./cm. All the indicated characteristics '70 to 76 were found experimentally, the processes according to the examples 77, '78, 79 representing smelting processes carried out with high power transformation.

It is also to be pointed out, that also within the limits holding good for the present process and above indicated, there are more favourable and more unfavourable values for the gas pressure p, not with respect to stability but as regards the efficiency of the process, that is to say economically. The most favourable value of the pressure can be ascertained for each process by ascertaining the voltage U in dependence upon the pressure p for a given constant temperature T. This fact, experimentally ascertained in many processes, seems to indicate that the dimensions of the cathode drop space immediately in front of the surfaces participating in the process have an influence on the efiiciency of the transformation of electrical into thermal energy on the surfaces participating in the process.

The above mentioned rule-ionic current always greater than thermal emission current-in order to avoid unstable ranges of the discharge characteristic, naturally holds good not only for the end state of the discharge but must be considered also as regards the starting process above explained. Of course, at the beginning of the starting operation, the regular thermal electronic emission is small owing to the generally low temperature of the surfaces participating in the processes, so that the mentioned rule can be normally maintained. It is however to be pointed out that, in addition to the regular thermal electronic emission, frequently a strong electronic current of individual oxidized surfaces or of surfaces otherwise rendered impure, can be emitted.

Such spontaneous electronic emissions arising simultaneously at different points may have such current densities that, in spite of a total current of, for instance, 0.1 ma./cm. at the corresponding point, the field distribution in the cathode drop space may be disturbed to a large extent, which may easily lead to locally limited are discharges, if the above mentioned special measures were not taken into consideration (series impedance in the supply circuit, control free of inertia for limiting the current). It is a feature of the above described process, that the discharge is supplied by a supply circuit having a variable series impedance. The value of such series impedance is to be made smaller than the impedance of the glow discharge path, i.e. the ratio between the voltage U and the supplied current. This is important specially for a process carried out with an energy density of 1 watt per square centimeter on the surface.

We claim:

1. Process for regulating the electrical supply circuit for the treatment of an object in a gas atmosphere in a discharge chamber by an electric glow discharge initiated and maintained by said supply circuit, said circuit having a series impedance and the object to be treated being connected in said circuit so as to operate at least part of the time as a cathode during the discharge and being heated by impinging ions while conversion of the glow discharge into an arc discharge is prevented, said process comprising reducing the said impedance to a value smaller than the impedance of the glow discharge path in the chamber and increasing the supplied electric energy to increase the ionic current impinging the cathode surfaces to such degree that at every point of the cathode surfaces such current is equal to or greater than the electronic current emitted at such point, regulating the voltage to a value over volts, and maintaining the ionic current density greater than one-tenth milliampere per square centimeter of surface.

2. A process according to claim 1, characterised by the use of a gas pressure p in mm. Hg, a total voltage U of the discharge gap, and a total current density i in amp/cm. of the surfaces participating in the process such that, at least in the end state of the discharge, in a diagram of the function (t EG the working characteristic of the glow discharge process lies within the region comprising all the values smaller and all the values smaller than 3. A process according to claim 2, wherein within the said limits the gas pressure p and the voltage U are varied in dependence on each other to obtain; the predetermined temperatures of the process, whereby the economically favourable values of U and p pursuant to the diagram U =f(p) are used for the operation.

4. A process according to claim 1, characterised by that use is made of an electrode output of at least 1 to 10 kilowatts total output.

5. Process according to claim 1, wherein the ionic stream density is made greater than the value defined by the straight line 68b in the diagram of FIG. 2.

6. Process according to claim 1, wherein the ionic stream density is made greater than the value defined by the straight line 680 in the diagram of FIG. 2.

7. Process according to claim 1, wherein the voltage is between 100 and 1000 volts.

8. Process according to claim 1, wherein the voltage is between 200 and 900 volts.

References Cited in the file of this patent UNITED STATES PATENTS 

